1. Field of the Invention
The invention relates in general to wireless communication systems, and more particularly, to overhead channel information delivery in wireless communication systems.
2. Description of the Related Art
In a cellular communications system, the area to be covered is divided up into a number of small areas called cells, with one base station (BS) positioned to give service or coverage to the mobile units within a cell. Each BS is connected to a base station controller (BSC) which is used to control several BSs and the provision of communication service to a mobile unit as it travels from one cell to another. The BSC's are in turn connected to a mobile switching center (MSC), which is generally a telephone exchange with special software to handle the mobility aspects of the mobile units. Most cellular communications systems consist of a number of MSCs each with their own BSs, and interconnected by means of fixed links. The MSCs interconnect to the public switched telephone network (PSTN) for both outgoing calls to and incoming calls from fixed telephones.
A mobile unit is connected, ultimately, to the MSC through over-the-air radio frequency (RF) reverse (mobile to BS) link to and the RF forward (BS to mobile) link from the BS. RF signals are exchanged between a respective mobile unit and one or more base stations over these links. Mobile units do not communicate directly with one another. Mobile unit to mobile unit communications across the span of one or more cells occurs through the base stations providing service to the mobile units desiring communication. Base stations communicate with a BSC using various media such as ground based wires or a microwave link, for example. The BSC can route calls to a PSTN through the MSC or can route packets to a packet switched network, such as the Internet. The base station controller also coordinates the operation of base stations within the system during handoff, for example.
A common feature of the RF forward and reverse links of many cellular systems is multiple users using a single communication channel occupying a limited frequency range. Cellular systems employ a variety of techniques for allowing multiple users to use a single communication channel occupying a limited frequency range. Code Division Multiple Access (CDMA) is one such technique. A popular standard for CDMA can be found in the TIA/EIA Standard TIA/EIA-95-B entitled “Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System.” In CDMA, multiple users can simultaneously use the whole communication channel without having to coordinate the timing of their transmission or the portion of the communication channel each user utilizes. More details on CDMA and spread spectrum communications can be found in, CDMA Principles of Spread Spectrum Communications, by Andrew J. Viterbi, Addison-Wesley Publishing Company, Reading, Mass., 1996. In contrast to CDMA, two other multiple access techniques either coordinate the timing of user transmissions, time division multiple access (TDMA), or the portion of the communication channel a user uses, frequency division multiple access (FDMA).
Regardless of the technique employed to provide multiple access, in a typical cellular system, the forward communications link includes one or more data channels, a pilot channel, and one or more paging channels, all of which are transmitted from the base station to mobile units. The pilot channel or pilot signal serves to define the boundaries of the cell area served by a base station. In a CDMA cellular system in accordance with IS-95, the pilot signal serves other purposes as well. For example, the pilot signal: provides for time reference and for amplitude and phase tracking; and allows mobile units to identify and become synchronized with the various base stations that are within range of their communication capability. The set of data channels carry the data associated with the various communication sessions (usually phone calls) and are directed to individual mobile units. These data channels are often called traffic channels. The paging channels are used by the base stations to notify mobile units when a request to communicate has been received. A paging channel is also used to transmit overhead messages. Overhead messages contain information that enables mobile unit-to-base station communication.
Overhead messages are not addressed to any specific mobile unit but are intended for distribution to each mobile unit within the corresponding coverage area. In an IS-95 CDMA cellular system, there are four types of overhead messages: system parameters, access parameters, channel list, and neighbor list overhead messages. Each type of overhead message must be broadcast at least once per second. For example, the neighbor list overhead message continually broadcasts a list of neighboring base stations through which communication may be possible. The mobile units use the neighbor list to monitor signals from the neighboring base stations in the event that the mobile unit enters the coverage area of neighboring base stations. Cellular systems employing other communications standards may have other types of overhead messages.
The overhead messages are typically received and processed by a mobile unit when a mobile unit is not engaged in a call or attempting to engage in a call, (i.e., when it is in an ‘idle state’). The term idle state is somewhat of a misnomer because the mobile unit can be very busy in the idle state. During the idle state, the mobile station periodically wakes up and listens to the paging channel and processes the messages on that channel. The overhead messages may remain the same for a substantial period of time during which the mobile station periodically wakes up and listens to the paging channel. Since it is not desirable to have the mobile unit wake up, receive the overhead message, and decode it only to determine that the overhead message is the same as the previous message that was decoded earlier, in the interest of conserving battery power, a sequence number is transmitted along with the overhead message. When the mobile unit wakes up, it receives the sequence number of the overhead message and decides whether to stay awake and receive the overhead message. In many instances, the mobile unit will go back to sleep after receiving the sequence number because the sequence number is the same as the sequence number received the last time the mobile unit woke up. Since listening for overhead messages requires a certain amount of power and since the listening function is performed frequently (as often as once per second), limiting the amount of time a mobile unit performs the listening function reduces the overall power consumption of that mobile unit and therefore increases the life of any battery or other power storage system utilized by that mobile unit. Significant power savings may occur because in many instances the overhead messages may remain the same for a substantial period of time.
In many cellular systems, the overhead messages and the sequence numbers to be transmitted by a base station are controlled from a central location such as a base station manager (BSM). One reason for providing this centralized control arises from the nature of the service typically provided by cellular systems. Cellular systems typically provide voice service or connection to and from a land based PSTN. A PSTN typically has a highly concentrated switching structure. As a result, cellular services desiring to interface to it have had to use central switches for routing calls. In many cellular systems providing voice service via a PSTN, phone calls are bundled at a central switch (MSC or MSCs) for transmission through the PSTN or unbundled for transmission through the wireless network. Consequently, due to the existence of a central location (BSM or MSC) at which links from many BSs terminated, it has been logical to take advantage of this central location to control remotely from the central location the programming of overhead messages and sequence numbers at each of the base stations in the cellular system. In contrast, cellular systems providing packet data service rather than voice service typically interface to a highly decentralized network. An example of a decentralized network is the internet. A cellular system providing packet data service provides a wireless link from the users of the cellular system to a router which links the users to the decentralized network. Technically, the internet is a collection of standard protocols, such as Transmission Control Protocol/Internet Protocol (TCP/IP), that enable dissimilar computer systems and networks to send and receive information. Some of the more popular protocols include: e-mail (electronic messaging), file transfer protocol (file transferring), World Wide Web (graphical interface utilizing links by hypertext documents), telnet (telephone network connections), IRC (Internet Relay Chat), gopher (information organization).
A message received from a mobile user of the packet data cellular system is forwarded by the base station to a router. The router breaks down the message into smaller pieces, know as packets. The packets travel separately across the network, but are ultimately re-formed together when all the pieces of the message arrive at its destination. During the trip, the packets merely bump along from one router to another router and through bridges and switches. No particular pathway is selected from the outset. Each router or switch looks at the destination address of the packet, but does not inspect its contents, and decides the best way to pass it along.
Since a router typically connects directly to the decentralized network and typically sends data along multiple paths to a destination there is often no need for a switch at a central location to set up a single path to and from a destination. As a result in a wireless data system providing access to a decentralized network there is not likely to be a central location, such as the BSM or MSC, which can also be used to control base stations and the update of overhead messages in the cellular system. Thus, there has been a need for overhead message delivery in a wireless communication system, such as a cellular system, providing access to a decentralized network. The present invention meets this need.